1. Field of the Invention
The present invention relates to a flexible printed board which is a base board of a flexible printed circuit board for establishing the electrical connection in, for example, an electronic apparatus. More particularly, the present invention relates to a flexible printed board which has an insulator in the form of a polyimide-resin layer formed on a conductor thereof More particularly, the present invention relates to an improvement in the bonding characteristic of a flexible printed board and prevention of curling.
2. Description of the Related Art
To reduce the size and cost of a so-called portable electric product, such as a portable recording/reproducing apparatus, the flexible printed circuit board is usually employed. The flexible printed circuit board establishes the connection among the electric circuits to one another, the flexible printed circuit board being a relatively low cost board which permits space saving. The conventional flexible printed circuit board has circuits formed on a conductor of a flexible printed board, which is a base board thereof, by etching the conductor.
The flexible printed board, which is the base board for the flexible printed circuit board, usually incorporates polyimide films because of satisfactory flexibility and heat resistance of the polyimide films. Specifically, a double-layer flexible printed board 50 structured as shown in FIG. 1 has been disclosed and put into practical use. That is, copper foil 51 is directly coated with varnish for a polyimide precursor, the varnish being made of polyamic acid. The polyimide precursor is dried and turned into an imide so that a polyimide-resin layer 52 is formed.
The foregoing double-layer flexible printed board 50, however, suffers from a problem in that curling undesirably occurs because the polyamic acid used to coat the metal foil 51 is turned into an imide at high temperatures.
It can be considered that the curling is caused from the difference between the thermal contraction ratio of the metal foil 51 and that of the polyimide-resin layer 52. The difference occurs after the temperature of the polyimide-resin layer 52 has been lowered to room temperatures after the imidation has been realized. The difference in the thermal contraction ratio is caused from the difference between the thermal expansion of the metal foil 51 and that of the polyimide-resin layer 52.
For the purpose of overcoming the curing and other drawbacks, Japanese Patent Laid-Open No. 8-250860 has disclosed a double-layer flexible printed board.
That is, the disclosed double-layer flexible printed board has a multi-layer structure with polyimide layers, whereby the difference in the coefficient of linear thermal expansion of the metal foil and that of polyimide-resin layer becomes low.
As a result, the double-layer flexible printed board has little curling on forming the polyimide-resin layer on the copper foil.
The foregoing method has the step of adjusting the coefficient of thermal expansion of the polyimide-resin layer including the copper foil. Thus, the foregoing method, however, encounters re-occurrence of curling after the flexible printed circuit board has been manufactured by forming a circuit pattern through etching of the copper foil which is performed after the flexible printed board has been manufactured. That is, the foregoing method cannot correct the curling after the circuit has been formed.
Another reason why the curling of the double-layer flexible printed board occurs is that the material is contracted when the polyimide precursor is turned into an imide. The contraction causes the above-mentioned curling to occur.
The phenomenon of the occurrence of curling which is caused from the contraction of the material occurring when the polyimide precursor is turned into an imide cannot effectively be prevented as yet. Therefore, the conventional double-layer flexible printed board cannot completely remove the curling. The curling causes the accuracy between the conductors of the circuit after the etching process has been completed to deteriorate. Thus, the double-layer flexible printed board cannot satisfactorily form a precise pattern which has been required to furthermore improve the degree of precision.
Since the contraction of the material occurs when the polyimide precursor is turned into an imide, distortion occurs between the copper foil 51 and the polyimide-resin layer 52. As a result, there arises another problem in that the bonding strength decreases.
The conventional double-layer flexible printed board has been manufactured as shown in FIG. 2 such that a film 53 formed by coating the surface of the copper foil with the varnish for the polyimide precursor and made of the polyamic acid is wound into a roll shape. Then, heat treatment is performed in the foregoing state to turn the varnish for a polyimide precursor into an imide.
Since the rolled material is subjected to the heat treatment when the imidation is carried out, the overall body of the rolled film 53 cannot uniformly be subjected to the heat treatment. As a result, the flexible printed board 50 unwound from the rolled state after the imidation has been realized suffers from the following problem: the bonding strength disperses between the copper foil 51 and the polyimide-resin layer 52 in a core portion 53a nearest the winding shaft, an intermediate portion 53b and an outer portion 53c. As a result, a uniform quality cannot be obtained. Specifically, the portion from the core portion 53a to the intermediate portion 53b suffers from instable bonding strength between the copper foil and the polyimide-resin layer.
In view of the foregoing, an object of the present invention is to provide a flexible printed board for improving the bonding strength and its non-dispersion.
Another object of the present invention is to provide a flexible printed board for improving curling and the dimensional stability for forming a fine circuit.
To achieve the above-mentioned objectives, according to one aspect of the present invention, there is provided a flexible printed board comprising:
at least one or more polyimide-resin layers formed on a conductor thereof, wherein
the polyimide-resin layer which is in contact with the conductor is obtained by forming, and turning into an imide, a polyimide precursor which contains polyamic acid components prepared owing to reactions of acid anhydride and amine and expressed by the following chemical formula (5) and polyimide components prepared owing to reactions of acid anhydride and isocyanate and expressed by the following chemical formula (6): 
where each of Ar1 and Ar2 is an aromatic group and n is an integer, 
where each of Ar3 and Ar4 is an aromatic group and in is an integer.
As described above, the flexible printed board according to the present invention incorporates the polyimide precursor which is the material of the polyimide-resin layer and made contact with the conductor contains the polyimide components expressed by chemical formula 6 as well as the polyamic acid components expressed by chemical formula 5.
The flexible printed board according to the present invention incorporates the polyimide precursor, which is the material of the polyimide-resin layer made contact with the conductor, is previously turned into an imide before the imidation is performed. Therefore, contraction of the material caused from the imidation of the polyimide precursor can be prevented. As a result, the flexible printed board according to the present invention is able to prevent distortion occurring between the conductor and the polyimide-resin layer which is made contact with the conductor when the imidation is performed. Therefore, the bonding strength between the conductor and the polyimide-resin layer can be increased.
Moreover, the flexible printed board according to the present invention is able to prevent contraction of the material when the rolling and imidation is performed. Therefore, dispersion of bonding strength between the conductor and the polyimide-resin layer can be prevented in the core portion, the intermediate portion and the wound portion in a state of the flexible printed board subjected to the imidation and the unwinding process have been completed. As a result, the quality can satisfactorily be uniformed.
Moreover, the flexible printed board according to the present invention is able to prevent contraction of the material caused from the imidation of the polyimide precursor. Therefore, curling can be minimized.
According to another aspect of the present invention, there is provided a method of manufacturing a flexible printed board comprising the steps of: forming a first polyimide-precursor-layer by coating a conductor with varnish for a first polyimide precursor containing polyamic acid components prepared owing to reactions of acid anhydride and amine and expressed by the following chemical formula 7 and polyimide components prepared owing to reactions of acid anhydride and isocyanate and expressed by the following chemical formula 8; coating the surface of the first polyimide-precursor-layer with varnish for a second polyimide precursor composed of polyamic acid components prepared owing to reactions of acid anhydride and amine; and subjecting the first polyimide-precursor-layer and the second polyimide-precursor-layer to heat treatment so as to turn the first and second polyimide precursors into an imide so that the first and second polyimide-resin layers are formed. 
where each of Ar1 and Ar2 is an aromatic group and n is an integer 
where each of Ar3 and A4 is an aromatic group and m is an integer
As described above, the method of manufacturing the flexible printed board according to the present invention employs the polyamic acid expressed by chemical formula 7 and the polyimide components expressed by chemical formula 8 as the polyimide precursor which is the material of the first polyimide-resin layer made contact with the conductor so that the precursor layer for the polyimide is formed.
That is, the method of manufacturing the flexible printed board according to the present invention employs the material previously turned into an imide before the imidation is performed as the polyimide precursor which is the material of the first polyimide-resin layer. Therefore, contraction of the material caused from the imidation of the polyimide precursor can be prevented. As a result, the method of manufacturing the flexible printed board according to the present invention is able to prevent distortion occurring between the conductor and the first polyimide-resin layer when the imidation is performed. As a result, the bonding strength between the conductor and the polyimide-resin layer can be increased.
The method of manufacturing the flexible printed board according to the present invention is able to prevent contraction of the material when the rolling and imidation is performed. Therefore, dispersion of the bonding strength between the conductor and the first polyimide-resin layer in the core portion, the intermediate portion and the wound portion of the unwound flexible printed board subjected to the imidation can be prevented. Thus, the quality can significantly be uniformed.
Moreover, the method of manufacturing the flexible printed board according to the present invention is able to prevent contraction of the material caused from the imidation of the polyimide precursor. Therefore, curling can be prevented.
Other objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.